This proposal is a continuation of work designed to exploit the special features that Drosophila sex determination appears to offer for understanding how information for cellular differentiation in higher eukaryotes is genetically encoded, how it can be disrupted by mutations, and how it may have been evolved. The goal of this study is to understand the specific genetic regulatory mechanisms that operate in this organism to govern the initiation and maintenance of sex-specific pathways of development. The gene called Sex-lethal occupies a central position in this process. Tests of several hypotheses regarding Sxl's regulation are presented. One of the most important is the proposal that Sxl produces a positive self-regulatory product that is required for the initiation and/or maintenance of the gene's own activity state. Another is the proposal that a genetic element called sisterless-a is part of a system of elements whose overall relative dose serves as a developmental signal to determine Sxl's activity state very early in development. Only a 50% difference in this parameter elicits all the differences between the sexes. The genetic context in which Sxl operates will be explored, including genes that act in the mother to control, in an epigenetic fashion, the behavior of genes in the zygote. Temporal and tissue-specific aspects of Sxl's own regulation, and of its regulation of subordinate genes and/or gene products will be investigated. As a byproduct of this study of sex determination, a considerable amount should be learned regarding the effects of mobile genetic elements on gene function and meiotic recombination in higher eukaryotes. Many of the most useful mutant Sxl alleles carry insertions and rearrangements of such mobile elements. This analysis will be carried out using the techniques of developmental genetics which have proven so powerful over the past eight years of this study. This genetic work will complement a parallel study of Sxl at the molecular level, a study that was made possible by the genetic analysis that preceded it. Information from the experiments proposed here should provide the background needed to extend molecular analysis of this system into important new areas.